Physical origin of threshold voltage problems in polycrystalline silicon/ Hf O₂ gate stacks

Based on theoretical calculations, we find that at p+ polycrystalline silicon (poly-Si) Hf O2 gates, Si interstitials are easily migrated from the electrode, forming Hf-Si bonds with a charge transfer to the electrode, and the resulting interface dipole raises the Fermi level of poly-Si toward the pinning level, causing high flat band voltage shifts. In O-rich grown Hf O2 on Si substrates, the Si atoms substitute for the Hf sites, leading to the formation of Hf-silicate layers, while under O-poor conditions, they remain as interstitial defects, binding with the Hf atoms, and behave as a negative-U trap, which causes the threshold voltage instability.